This is a ready-to-use worksheet for calculating the log reduction of a disinfectant or sporicide on a carrier (coupon), from the recovered counts. Its purpose is to make the calculation transparent and to force the one check that most weak studies skip: the log reduction is computed against the recovered count on the untreated control, not the theoretical inoculum, so a poor recovery method cannot make a weak disinfectant look strong. Replace every <<FILL: ...>> placeholder. A filled specimen follows. This is general guidance to adapt, not legal or regulatory advice.
Purpose
Compute the log reduction for one organism on one surface at one contact time, document the recovery efficiency, and confirm the reported reduction is arithmetically possible given that recovery.
The method in one line
Log reduction = log10(recovered count on the untreated control) minus log10(recovered count after treatment). Both counts are what the recovery method actually recovers, not what was put down.
Field definitions
| Field | Meaning |
|---|---|
| Inoculum applied | CFU placed on the coupon before drying |
| Untreated control recovered | CFU recovered from an inoculated, untreated, dried coupon (the real starting count) |
| Recovery efficiency | Untreated control recovered / inoculum applied, as a percentage |
| Treated recovered | CFU recovered from a coupon after the disinfectant and contact time |
| Log reduction | log10(untreated control recovered) minus log10(treated recovered) |
| Maximum demonstrable reduction | log10(untreated control recovered) minus log10(limit of detection); the most the study can show |
The worksheet
Header
| Field | Entry |
|---|---|
| Study / protocol reference | <<FILL>> |
| Organism | <<FILL>> |
| Surface material | <<FILL>> |
| Contact time | <<FILL>> |
| Replicate | <<FILL>> |
| Analyst / date | <<FILL>> |
Counts and calculation
| Line | Value |
|---|---|
| 1. Inoculum applied (CFU) | <<FILL>> |
| 2. Untreated control recovered (CFU) | <<FILL>> |
| 3. Recovery efficiency = line 2 / line 1 x 100 | <<FILL>> percent |
| 4. log10 of untreated control recovered (line 2) | <<FILL>> |
| 5. Treated coupon recovered (CFU) | <<FILL>> |
| 6. log10 of treated recovered (line 5) | <<FILL>> |
| 7. Log reduction = line 4 minus line 6 | <<FILL>> |
| 8. Limit of detection (CFU) and its log10 | <<FILL>> |
| 9. Maximum demonstrable reduction = line 4 minus log10(LOD) | <<FILL>> |
Checks
| Check | Result |
|---|---|
| Recovery efficiency adequate for the intended claim | <<FILL: Yes/No; if No, fix recovery before concluding>> |
| Reported reduction (line 7) is less than or equal to the maximum demonstrable (line 9) | <<FILL: Yes/No>> |
| Neutralization controls passed for this run | <<FILL: Yes/No; if No, count is not usable>> |
| Meets the acceptance criterion for the organism class | <<FILL: Yes/No>> |
Acceptance criteria
The log reduction is valid when the recovery efficiency is documented and adequate, the neutralization controls passed for the run, the reported reduction does not exceed the maximum the recovered control could demonstrate, and the reduction meets the predefined criterion for the organism class. If a “no-survivor” result is claimed, the reduction is reported as “greater than or equal to” the maximum demonstrable value, never as an exact number the study could not resolve.
Filled specimen
Illustrative, Bacillus subtilis spores on polycarbonate at a 4-minute contact time.
| Line | Value |
|---|---|
| 1. Inoculum applied | 1.2 x 10^6 CFU |
| 2. Untreated control recovered | 9.4 x 10^5 CFU |
| 3. Recovery efficiency | 78 percent |
| 4. log10 of untreated control recovered | 5.97 |
| 5. Treated coupon recovered | 3.8 x 10^2 CFU |
| 6. log10 of treated recovered | 2.58 |
| 7. Log reduction | 3.39 (about 3.4) |
| 8. Limit of detection | 10 CFU, log10 = 1.0 |
| 9. Maximum demonstrable reduction | 4.97 |
Checks: recovery 78 percent, adequate and documented; reported reduction 3.4 is less than the maximum demonstrable 4.97, so it is arithmetically sound; neutralization controls passed; criterion for spores (at least 3.0) met. Result: pass.
The worksheet catches the classic error before it ships: if someone had claimed a 6-log reduction here, line 9 shows the recovered control (about 10^6) could demonstrate at most about 5 logs down to the detection limit, so a 6-log claim is impossible and the recovery or the counts are wrong.
Common findings this worksheet prevents
- Log reduction computed against the theoretical inoculum instead of the recovered control count, overstating performance.
- A recovery efficiency never stated, so the reduction cannot be judged.
- A claimed reduction larger than the recovered control could arithmetically demonstrate.
- A kill number reported from a run whose neutralization controls failed.
How to adapt this worksheet
- Use one worksheet per organism, surface, and contact time, and average replicates on the qualification report.
- Set the limit of detection from your recovery and plating method.
- Pair this with the qualification protocol and the coverage matrix.
- Where a validated spreadsheet performs the calculation, confirm it uses the recovered-control basis and reproduce one result by hand as a check.